In 1950 Twin Coach announced its large 52-S model and in Summer that year the demonstrator came to Chicago. CTA officials, impressed by new bus, promptly placed an order for 500 propane-powered 51-seats units - the largest single urban bus order in history up to that time. Deliveries began in December 1950, opening the propane era in Chicago and first bus was placed in service in January 1951 on route 82A/Kedzie-Homan. The buses were numbered 5000-5499 and for many years they were the symbol of CTA modern equipment. They were retired from the main routes in late 60s and all remaining buses were withdrawn in early 70s.